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Rietveld Refinement.

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Presentation on theme: "Rietveld Refinement."— Presentation transcript:

1 Rietveld Refinement

2 Why do Reitveld Refinement?

3 What are the limitations of Rietveld?
Phase detection limit of 1-5% Impurity atom determination New/unknown crystal structures Determination of disorder parameter Multiple parameters can have same effect diffraction patterns Equipment (powder vs. single crystal, x-rays vs. neutrons) Garbage in, garbage out! (noisy data, poor quality sample, poor sample prep)

4 Basic refinement procedure
Background Lattice parameters Peak intensities Peak shapes Peak positions Phase fractions Experimental diffraction pattern Starting crystal structure (.cif, ICSD) Refinement program: GSAS, FullProf Assess: Goodness of fit/R factors Impurity phases Peak/background shapes Difference pattern Instrument file (.inst, LaB6 standard)

5 Some common challenges/problems
False minimas Refinement diverges (“blows up”) Which goodness of fit to choose? R vs. Chi sq? Preferred orientation Large amorphous background Incorrect starting crystal structure or lattice parameter Poor quality data!

6 Resources and links GSAS/EXPGUI: GSAS tutorials: FullProf: PowDLL (file conversion): Lots of info about fitting parameters: People: Lauryn Baranowski Lakshmi Krishna

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